LISP provides routing scalability by separating the identifier of an endpoint device from the locator of the endpoint device. LISP-alternative topology (LISP-ALT) is a hierarchical aggregation of endpoint identifiers (EIDs) to provide a distributed EID-to-routing locator (RLOC) mapping database. An ingress tunnel router (ITR) hosts a map cache. A map resolver (MR) may act as a first-hop LISP-ALT router. The MR has generic routing encapsulation (GRE) tunnels to other LISP-ALT routers, uses border gateway protocol (BGP) to learn paths to egress tunnel routers (ETRs), and uses path information to forward map-request (mReq) messages over LISP-ALT to ETRs.
An EID is a 32-bit (for Internet protocol (IP) version 4 (IPv4)) or 128-bit (for IP version 6 (IPv6)) value used to identify the ultimate source or destination for a LISP-encapsulated packet. A LISP header includes source and destination EIDs. A destination EID may be obtained using the domain name system (DNS). EIDs are globally unique.
A map server (MS) provides an authoritative mapping of EIDs to locations. An ETR, pre-configured with the MS using a shared secret or other authentication information, is a trusted routing destination for the EIDs mapped to it. EIDs may be mapped to multiple ETRs.